This invention is a continuation-in-part of U.S. patent application Ser. No. 747,488 filed June 21, 1985, titled "Fiber Optic pH Sensor Having Low Drift Rate", which application is incorporated herein by reference.
This invention relates generally to fiber optic sensors, and more particularly to CO.sub.2 sensors. A common method of measuring the partial pressure of carbon dioxide is based on the use of a pH sensor. Typically, the pH sensing material is placed in contact with a solution containing a bicarbonate. The partial pressure of carbon dioxide is determined by fixing the concentration of bicarbonate and then measuring the pH of the solution, which will be proportional to the partial pressure of CO.sub.2. The pH-sensing material may be a glass pH electrode or, more simply, a pH-sensing dye.
Fiber optic pH-sensors are based on the principle that certain materials' optical properties change with pH. For example, D. Lubbers et al., "Nanoencapsulated Fluorescence Indicator Molecules Measuring pH and PO.sub.2 Down to Submicroscopical Regions on the Basis of the Optode-Principle", Z. Naturforsch., 32c, 133-134, 1977, used a fluorescent material (.beta.-methyl umbelliferone) encapsulated in polymer beads having porous outer membranes to measure pH. The fluorescent intensity of the material is proportional to pH. Peterson et al., "Fiber Optic Probe for Physiological Use", Anal. Chem., 52, 864-869, 1980, used a colorimetric pH-indicating dye (eg. phenol red) bound to polyacrylamide hydrogel by copolymerization of the dye with the acrylamide monomer in the preparation of their fiber optic pH-sensing probes. The dye changes color with changes in pH, and the color change corresponds to a change in the amount of light of a specified wavelength absorbed by the dye (e.g. green light in the case of phenol red). The Peterson et al. fiber optic probe (also described in U.S. Pat. No. 4,200,110) consists of an ion permeable membrane which encloses the distal ends of a pair of optical fibers. Retained within the hollow membrane and distal to the distal ends of the optical fibers is the pH-indicating sensor (dye/gel polymer). One fiber provides light to the sensing gel and the other fiber transmits the absorbable light from the sensing gel. Thus, the pH of a solution placed in contact with a column of sensing gel, through the ion permeable membrane, can be determined by the intensity of the absorbable light transmitted through the optical fibers.
A problem associated with pCO.sub.2 sensors, formed from a pH sensor such as that suggested by Peterson, employing phenol red dye, is leakage of ions across the ion-permeable membrane during the process of achieving equilibrium of the pCO.sub.2 between the measuring solution and medium. It has also been difficult to fix the dye/bicarbonate concentrations in the gel used by Peterson. G. G. Vurek proposed a solution to this problem in his U.S. Pat. application Ser. No. 470,920. Vurek's pCO.sub.2 sensor includes the same optical arrangement as Peterson, but uses a dye/water/bicarbonate solution contained in a gas-permeable barrier such as silicone rubber. While Vurek's sensor offers advantages in terms of ion impermeability over Peterson's sensor, Vurek's sensor lacks stability with respect to drift and has a slow response time.
pH sensors and pH-based sensors which are used for applications such as continuous patient monitoring are typically manufactured, calibrated, and then sold for use. Once used, the sensors are thrown out. pH sensitive dye/gels change color or fluorescence as a result of changes in the pH of the solution. For analytical purposes it is critical that the color or fluorescent intensity of the dye/gel be constant for a given pH. Sensors made with the conventional dye/gel polymer (such as phenol red/polyacrylamide) show a substantial drift in measured pH with time, resulting from a continual decay in the absorbance of the dye/gel. This drift also corresponds to an increase in the protonation constant, K, of the gel, and severely limits the accuracy of the sensors for pH measurements over extended periods of time (i.e., more than a few hours), or necessitates frequent recalibration of the probes. Furthermore, the initial optimum operating range of these sensors is variable and also drifts with time because of changes in K. All of these factors contribute to a serious lack of reliability in pH-sensors which impedes their use for continuous patient monitoring.
Therefore, it is an object of the present invention to provide a pCO.sub.2 -sensor having minimal drift and fast response time.
It is another object of the present invention to provide a dye/gel polymer, having a consistently repeatable K value, suitable for use as a pCO.sub.2 -sensor.
It is yet another object of the present invention to provide a chemically stable pCO.sub.2 -sensing probe, having minimal drift, suitable to be implanted in tissue for physiological studies.
It is also an object of the present invention to provide an optically stable pCO.sub.2 -sensing probe, i.e. one in which the color or fluorescence is stable for a given pH and pCO.sub.2.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.